Short range Doppler radars use a homodyne system, in which the received signal is mixed with the transmitted signal. Mixing the received and transmitted signals results in a low frequency output that is proportional to the Doppler shift between the received signal and the transmitted signal. However, since the mixing process uses the transmitted signal, isolating the received signal from the transmitted signal is difficult.
FIG. 1 shows a conventional simple Doppler radar system 100 for measuring the velocity of a radar signal relative to the ground. The transmitted signals 104 are generated by an oscillator 110 and transmitted through an antenna 112. The transmitted signals reflect of the target (e.g., the ground) and produce reflected signals 102 which are received at the antenna 112. The transmitted signals 104 is split into a transmitted in-phase (“I”) signal 104a and a transmitted quadrature (“Q”) signal 104b, where the I signal 104a is ninety degrees out of phase with the Q signal 104b. Similarly, the received signals 102 include a received I signal 102a and a received Q signal 102b. The received I signals 102a and the transmitted I signals 104a are mixed at an I mixer 106. The received Q signals 102b and the transmitted Q signals 104b are mixed at a Q mixer 108. Differences between the I signals 102a and 104a and between the Q signals 102b and 104b are used to determine the direction of travel of the radar system 100.
In the Doppler radar system 100, the transmitted signals 104 are not separated from the received signals 102. Thus, the received signal 102 cannot be independently amplified before it is sent to the mixers 106 and 108. The power of the received signal 102 is low, and is then further divided between the I mixer 106 and the Q mixer 108. Furthermore, the I mixer 106 and the Q mixer 108 are both very lossy. This results in a low Signal to Noise Ratio (“SNR”). In the radar system 100, there are two ways to improve the SNR. One way to improve the SNR is to increase the transmitted signal power (thereby increasing the received signal power). However, increasing the transmitted signal power causes the radar system 100 to draw more DC power and may raise safety concerns. The other way to improve the SNR is to increase the antenna gain by using a larger antenna; however this adds size and cost to the system.
Accordingly, it is often desirable to isolate the received signals 102 from the transmitted signals 104. To achieve isolation between the transmitted signal 104 and the received signal 102 in a homodyne radar, one existing solution is to use two separate antennas: a receive antenna and a transmit antenna. FIG. 2 is a schematic diagram of a Doppler radar system 120 with a separate receive antenna 122 and transmit antenna 124. As shown in FIG. 2, the transmitted signal 104 is generated at the oscillator 110, and then divided at a first power divider 132. Part of the transmitted signal is directed through the transmit antenna 124, and the other part is further divided at a second power divider 134. The second power divider 134 divides the transmitted signal 104 into a local oscillator I signal 104a and a local oscillator Q signal 104b. The local oscillator I signal 104a is sent to a balanced I mixer 136 and the local oscillator Q signal 104b is sent to a balanced Q mixer 138. The received signal 102 is received through the receive antenna 122 and sent to a low noise amplifier 126 for amplification. The amplified received signal 102 is then divided into a received I signal 102a and a received Q signal 102b at the power divider 128. The received I signal 102 is sent to the balanced I mixer 136 and the received Q signal is sent to the balanced Q mixer 138.
Another way to isolate the received signal 102 from the transmitted signal 104 in a homodyne radar is to use a circulator. FIG. 3 is a schematic diagram of a Doppler radar system 150 including a circulator 152. The circulator 152 separates microwave signals based on their direction. The circulator 152 includes three ports: one “transmit in” port for the transmitted signal 104 from the oscillator 110, one combined “transmit-out/receive-in” port for sending the transmitted signal 104 to the antenna 112 and for receiving the reflected signal 102 from the antenna, and one “receive out” port for sending the received signal 102 to the low noise amplifier 126. The circulator 152 provides isolation between the transmitted signal 104 and the received signal 102.
Another way to isolate the received signal 102 from the transmitted signal 104 in a homodyne radar is to use a Wilkinson power divider. FIG. 4 is a schematic diagram of a Doppler radar system 170 including a Wilkinson power divider 172. The Wilkinson power divider 172 separates the received signal 102 from the transmitted signal 104. In a similar design aimed at isolating received signals from transmitted signals in RFID systems, a branch line coupler has been used instead of the Wilkinson power divider 172. Both of these approaches attenuate the received signal significantly, reducing the SNR of the system.